A time varying harmonically modulated flow method, analysed with a two rate constant surface reaction model, provides a series of permeation, P_m, diffusion, D, and surface rate coefficients, k₁, k₂, for hydrogen passing through 304 and 316 stainless steel. The method developed here allows the evaluation of a consistent set of bulk metal values of D and P_m, in the temperature range 502 < T/K < 963, despite wide ranging surface conditions, and are described by:

D₃₀₄=(1.22±0.06)x10-⁸exp{[-(6.596±0.049)x10³/T]/K^-1} m²s^-1

P_m304=(4.82±0.21)x10-⁷exp{[-(7.990±0.044)x10³/T]/K^-1} molm 1s^-1Pa^-1/2

D₃₁₆=(7.28±0.94)x10-⁷exp{[-(6.296±0.109)x10³/T]/K^-1} m²s^-1

P_m316=(8.09±0.70)x10-⁷exp{[-(8.189±0.076)x10³/T]/K^-1} molm^-1s^-1Pa^-1/2

When surfaces are treated to form a stable oxide layer, the surface reaction rate constants are found to be interpretable in terms of molecular flow of hydrogen through the oxide. The permeation coefficients, P_mox, describing this flow are as follows:

P_mox3o4=(1.43±0.29)x10^-11exp([-7.661±0.081)x10³/T)/K^-1) molm^-1s^-1Pa^-1

P_mox3ls=(5.36±2.68)x10^-11exp{[-6.967±0.228)x10³/T)/K^-1) molm^-1s^-1Pa^-1